In the field of marine seismic survey operations, for example for oil and gas exploration at sea, it is known to employ a so-called reflection seismology method in order to determine, at least approximately, properties of the Earth's subsurface from reflected seismic waves. In this respect, the earth is a layered structure, each layer having different acoustic propagation properties. In a marine environment, a seismic vessel tows a sound source and one or more streamers having hydrophones disposed therein. Periodically, the source produces a high energy impulse-like signal, some of which propagates down through the sea water column, penetrates the seabed and continues propagating through the Earth's subsurface layered structure. Some of the energy of the signal is reflected at each boundary or interface between layers, known as reflection events, and travels back up through the Earth and seawater to the hydrophones. The signals arriving at each hydrophone are a convolution of the transmitted signal and the reflection events. The hydrophones translate received acoustic energy of the transmitted signal and the reflection events into electrical signal data. On the seismic vessel, the signal data generated by each hydrophone is recorded as a time series and, together with geometric information, is processed using a de-convolution filter to image the structure of the part of the Earth being surveyed.
Signals that reflect off the seabed and at interfaces of the subsurface layered structure are known as “primary reflections”. Data generated by the hydrophones in respect of these primary reflections enable the properties of the Earth's subsurface to be estimated. However, a significant amount of the sound energy emitted by the sound source travels within a space between the sea surface and the seabed and successively reflects off the sea surface and the other interfaces. For a given hydrophone, these signals, known as sea-surface multiples, arrive at the hydrophone by a number of different paths, sometimes referred to as “modes”. The direct, or incident, wave signal from the source to the hydrophone and the sea-surface multiples are added to the primary reflection signals. The direct wave signal from the source to the hydrophone and the sea-surface multiples therefore interfere with the wanted primary reflection signals and degrade an image of the Earth's subsurface layered structure. It is therefore necessary to model the interfering signal at each hydrophone so that the interfering signal can be subtracted from the wanted primary reflection signals and thereby enhance the image of the Earth's subsurface layered structure. To this end, a position estimate of the sound source at an instant of transmission of the acoustic signal and a position estimate of each hydrophone at each sampling instant are available from location determination and computing hardware aboard the seismic vessel in order to facilitate estimation, for each sea-surface multiple, of the time of flight of an acoustic signal from the sound source to each hydrophone.
“Marine seismic wavefield measurement to remove sea-surface multiples” (A. M. Ziolkowski, et al., Geophysical Prospecting, Volume 47, Number 6, November 1999) describes a typical method for removing sea-surface multiples and the incident wave. The method uses an array of hydrophones to make source measurements of a wavefield made during data acquisition. The incident field inferred by these measurements is removed leaving a scattered field response. Using a zero pressure condition at the sea surface, the effect of the sea surface multiples is removed. Good estimates of the corrected sound speed in water are however necessary for the method to be effective.
Ray tracing is a well known method to generate a synthetic multiple wave and subtract it from an actual wave to obtain supposedly multiple-free data. A first stage of this technique is to derive a sound speed profile by measuring the sound speed at a number of sample points through the water column to enable calculation of the sound speed as a function of depth. Then, using the laws of refraction and reflection, the paths or modes that a signal can take from the sound source to the hydrophone are traced and the travel time and distance traveled estimated for each mode. The data may be processed so that a corrected sound speed can be estimated as a function of horizontal distance between the sound source and hydrophone, the depth of the sound source and the depth of the hydrophone. The corrected sound speed may be used to convert a travel time measurement into a true distance.
In order to calculate the sound speed profile, it is known to measure the sound speed at each sample point using a direct reading instrument employing a so-called “sing-around” method. This type of instrument uses an ultra high frequency projector and a reflector at either end of a very stable baseline. The time of flight along repeated traverses of the baseline is measured and therefore, as the baseline length is known, the sound speed can be estimated. An example of the direct reading instrument using this principle is the MIDAS Sound Velocity Profiler available from Valeport Limited, UK. Alternatively, the sound speed can be estimated using an instrument that measures parameters such as conductivity, temperature and pressure depth and uses an empirically derived algorithm to calculate the sound speed at each sample point. An example of such an instrument using this principle is the MIDAS CTD Profiler available from Valeport Limited, UK.
However, both of these two techniques have significant error budgets associated with calibration, parameter estimation and/or the algorithm employed. For example, the sound speed profile within a given volume of seawater varies significantly throughout the day and from day-to-day. Furthermore, from a practical perspective, measurement of the sound speed profile is difficult, especially in deep water from a moving vessel. Repeating such measurements sufficiently regularly is therefore an onerous and costly exercise.
As an alternative method, it is known to adapt an Inverted Echo Sounder (IES) principle used by ocean physicists to investigate the stratification of the ocean. The IES is deployed on the seabed and projects an impulse-like acoustic signal in a narrow beam to the sea surface. The IES detects the reflected acoustic signal and records a time of flight associated therewith. Using a pressure sensor or by a number of other means, the depth of the water through which the acoustic signal propagated can be estimated. However, there are a number of practical problems associated with this method, for example loss due to the reflectivity of the sea surface, the “roughness” of the sea surface and the resulting incoherence of the reflected acoustic signal, aeration at the sea surface in rough weather and a two-way spherical spreading loss. These factors limit the ability to measure the water depth, the precision of measurement and the battery life of the instrument incorporating the IES. Also, implementation of a so-called “bi-static measurement path” by employing two IESs spaced apart on the seabed is difficult due to the need to synchronise each instrument incorporating a respective IES. In addition, the respective beams of the IESs have to be non-directive rather than directive beams, which is problematic. Problems are also associated with steering a beam in the correct direction at each instrument and a reflection loss at the surface, which increases as the angle of incidence at the sea surface reduces. Also, the data collected can be substantial, and for practical and commercial reason has to be recovered from a moving vessel, which constitutes a further complication. Furthermore, by way of contrast with a marine seismic survey vessel, using state of the art equipment, the Marine Seismic survey vessel can continue to operate and acquire useful data in a sea state as high as sea state 6. However, the IES method is unlikely to deliver useful data in this sea state.